Как получить информацию о подписке во флаттере

Вы можете использовать RevenueCat и Qonversion.

• RevenueCat

  try {
  PurchaserInfo purchaserInfo = await Purchases.getPurchaserInfo();
  // access latest purchaserInfo
  } on PlatformException catch (e) {
  // Error fetching purchaser info
  }
  

• Qonversion

  final Map<String, QPermission> permissions = await Qonversion.checkPermissions();
  final main = permissions['main'];
  if (main != null && main.isActive) {
  switch (main.renewState) {
    case QProductRenewState.willRenew:
    case QProductRenewState.nonRenewable:
      // .willRenew is the state of an auto-renewable subscription 
      // .nonRenewable is the state of consumable/non-consumable IAPs that could unlock lifetime access
      break;
    case QProductRenewState.billingIssue:
      // Grace period: permission is active, but there was some billing issue.
      // Prompt the user to update the payment method.
      break;
    case QProductRenewState.canceled:
      // The user has turned off auto-renewal for the subscription, but the subscription has not expired yet.
      // Prompt the user to resubscribe with a special offer.
      break;
    default: 
      break;
  }
  }
  

Если вам нужна дополнительная информация, обратитесь к их руководству.